In large pot, combine cream, milk, garlic, onion, 5 sprigs thyme, bay leaves (if using), salt and nutmeg. Bring to a low boil over medium heat; remove from heat and cover. Let stand for 45 minutes.
Bring cream mixture back to a low boil over medium heat for 5 minutes, then strain into a large bowl.
Preheat oven to 350°F. Meanwhile, peel and, using a mandoline, slice potatoes into ⅛ in (3 mm) thick slices and add to cream mixture, stirring to prevent them from sticking together.
In bowl, combine Gruyère and Parmesan cheeses. Set aside.
Spread butter on bottom and up sides of 2-L baking dish. Layer one-quarter of the potatoes, overlapping; sprinkle one-third of the cheese mixture overtop and sprinkle with ¼ tsp of the pepper. Repeat two more times, setting the extra cheese aside. Pour remaining cream mixture overtop and sprinkle with the remaining ¼ tsp pepper.
Tightly cover with aluminum foil; place on baking sheet and bake for 60 to 70 minutes. Remove foil; sprinkle with remaining cheese mixture and return to oven. Bake until potatoes are tender and top is golden brown, 20 to 25 minutes. Broil for 2 to 3 minutes for a deeper browning, if desired. Let rest for at least 30 minutes before serving.